Electric resistor



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S. J. SMITH ELECTRIC RESISTOR Filed July 21, 1943 Oct. 16, 1945.

Patented Oct. 16, 1945 I ELECTRIC RESISTOR Stanley .iames Smith, London, to Simmonds Aerocessorles England England, assignor Limited, London,

Application July 21, 1943, Serial No. 495,655 In Great Britain July 21, 1942 6 Claims.

This invention relates to electric resistors for use with alternating currents more particularly of high radio frequency. The residual reactive parameters of such resistors may considerably modify their performance and in many measurements, for example where the magnitude of a high frequency current is measured by measuring the voltage drop set up across a known resistor with a valve voltmeter, it is clearly important that these parameters should be as small and as constant as possible if the measurement is to have any real significance. Hitherto a short, thin and straight wire has usually been used as the resistor but even such a resistor may produce an error of 50% at M. c. s. due to the residual inductance. i

It is an object of the present invention to provide an improved electric resistor having a very low reactance in particular a very low inductive reactance.

A: further object of the invention is to provide a resistor which may be adequately screened and which shall be capable of dissipating considerable power at high radio frequency.

Qther objects and advantages of the present invention will be apparent from the following description with reference to the accompanying drawing in which Fig. 1 shows a perspective view of one form of the improved resistor, the component parts thereof being shown separated for greater clarity, and Fig. 2 shows in diagrammatic form an application of the resistor of Fig. 1 in a Q meter.

Referring to Fig. 1 the resistor comprises a relatively thin disc i of resistive material which is clamped round its periphery between two cup shaped housings 2 and 3 of brass, copper or other suitable high conductivity metal, which are secured together by means of screws passing through apertures 4 formed in the housing 2 and through apertures 5 formed in the disc I and threaded into tapped holes 6 formed in the housing 3. The ends of the housings are bored and tapped to receive the threaded ends of tubular members 1 and 8. A conductor 9 makes contact with a small central area 59 of the disc I of resistive material, and extends through the members 1 and 8 from which it is insulated as by means of beads of insulating material or in any other suitable manner.

In operation, radio frequency current is fed from one end of the conductor 9 to the centre of the disc I, from there flows radially to the periphery of the said disc and thence to the housing. The potential difierence set up between the Eli centre and periphery of the disc may be measured by connecting a voltmeter between the other end of the conductor 8 and the tubular member 8. The housings 2 and 3 and the tubular members 1 and 8 may be grounded and thus efl'ectively screen the resistor. I

The resistive material or which the disc I is formed will be chosen in accordance with the.

value of resistance desired. For resistors of low value it is preferred to provide a disc of insulating material such as mica which is coated as by sputtering or evaporation with a film of metal such as silver on one or both sides.

Referring to Fig. 2 there is shown in diagrammatic form the elements of a measuring instrument known as a Q meter or circuit magnification meter in which the improved resistor. is employed. @ohductor 9 extends between a terminal I0 and through the resistor H and screen 82 to a source of radio frequency current ii. The housing oi the resistor ii is mounted on and grounded to a metal panel ll by means of bracket i5. Terminal i6 is connected with the rotor of a variable condenser i! and one terminal of a vacuum tube voltmeter l8. The stator of the condenser i1 and the other terminal of the voltmeter ii are grounded to the panel H. In operation an inductance coil to be tested is connected between terminals l0 and I6. Radio frequency current of'known magnitude is fed from the source i3 through resistor ii and the potential difference set up across this resistor is applied in the known manner in series with the inductance to be tested and. the tuning condenser H. The voltage set up across the condenser 41 when the circuit is at resonance is measured by the voltmeter I8,

and the Q factor of the inductance under test is given by the ratio of this voltage to the voltage injected into the circuit.

As is well known-in the art such measurements are only reliable at high frequency when the resistor used has a very low reactance. The improved resistor of the present invention can be constructed to have a substantially negligible inductive reactance at frequencies as high as M. c. s. thus permitting accurate measurement in this range to be obtained. Moreover when the resistor comprises a metallic film this film may be made thin enough to avoid skin effects whilst owing to its relatively large area and the good thermal conductivity to the housing the resistor is capable of dissipating considerable power.

I claim:

1. A resistor comprising a disc or resistive material, a first terminal mounted at the centre of said disc, a second terminal comprising a cup shaped member to the .rim of which the periphcry of the said disc is secured, the said member being formed with an aperture in the base thereof to receive a tubular screening member-through which a lead to the said first terminal may extend normal to the plane of the said disc.

2. A'resistor comprising a disc of resistive material, a first terminal mounted at the centre of said disc, at second terminal comprising a pair oi recessed clamping members between which the periphery of said disc is secured, each or the said clamping members being formed with an aperture in which a tubular screening member is fixed, the said screening members extending outwardly of the clamping members normal to the plane of the disc and coaxially with said first terminal 4. A resistor comprising a disc oi resistive meteriai, a first terminal mounted at the center of said disc, a second terminal comprising a pair of disc-like members each having on one side thereof a peripheral flange secured together with the periphery of the said resistive disc clamped between the said flanges, at least one of the said disc-like members being formed with a central aperture through which a lead to said first terminal may pass.

5. A resistor comprising a disc of mica having formed thereon a uniform metallic layer, a first terminal mounted on the center or said disc in contact with the said metallic layer and a second terminal consisting of a disc oi conductive material having a peripheral flange the periphery oi said mica disc and the metallic layer thereon be ing secured in contact with said flange.

6. A resistor comprising a disc oi insulating material coated on et least one side with a uniform layer of resistive material, a first terminal mounted at the center of said disc, a second ter-- minal consisting of a disc of conductive material having a peripheral flange and means for securing the periphery of said insulating disc with the layer of resistive material thereon in ccntmt with said flange.

STANLEY JAWS Sh HTH. 

